Water Treatment
The process train
Conventional surface-water treatment runs coagulation, flocculation, sedimentation, filtration, and disinfection. Know what each step does and the operator's control lever for it: coagulant dose and pH for coagulation (rapid mix, seconds); mixing energy and time for flocculation (gentle, minutes); detention time and surface overflow rate for sedimentation; media, loading rate, and backwash for filters; concentration times time (CT) for disinfection.
Numbers examiners test
Turbidity for conventional filtration: combined filter effluent at or below 0.3 NTU in at least 95 percent of monthly readings and never above 1 NTU. Dose conversion: lb/day = mg/L x MGD x 8.34. CT = residual (mg/L) x contact time (minutes); the required CT rises as pH and temperature fall.
Traps
Do not confuse coagulation (destabilizing particles, seconds of rapid mix) with flocculation (building settleable floc, minutes of gentle mixing). Do not use a CT table value for the wrong temperature; cold water needs more CT, not less. When reading a jar test, pick the dose that meets the turbidity goal at the lowest chemical cost, not simply the fastest-settling jar. Remember that chlorine demand is satisfied before any free residual appears.
Worked example
A plant holds 1.2 mg/L free chlorine with 45 minutes of contact time at peak flow. CT = 1.2 x 45 = 54 mg/L-min. If the required CT at that pH and temperature is 60, the plant is short and must raise residual, add contact volume, or cut flow. At 2 MGD, an alum dose of 15 mg/L works out to 15 x 2 x 8.34 = 250 lb/day.